1. Field of the invention
This invention relates to the proving of flow meters and to various improvements in the design, construction, and operation of meter provers.
Flow meters to be proved provide an output signal in relationship to the fluid flow. The output signal may be digital or analog. It is desirable to know the relationship of signal to flow as precisely as possible and this relationship changes over time. Therefore, it is desirable to calibrate or prove the meters to define the relationship between the output signal and the flow.
There are several ways to prove meters. The class of provers that this invention relates to are unidirectional compact provers. These provers all have an inlet, a cylindrical flow conduit some of which is a measuring section, a movable fluid barrier or piston, a valve, two detectors for detecting the piston when it is at each end of the measuring section and an outlet. The fluid flow path is through the meter, the inlet to the conduit, the cylindrical flow conduit with its measuring section, and, thence, out the outlet. The piston reciprocates upstream and downstream in the cylinder. In the upstream direction, the valve is open providing a path for the fluid flow. In the downstream direction, which is the prove stroke, the valve is closed and the piston moves with the fluid flow. The detectors signal when the piston enters and leaves the measuring section. This in conjunction with the output signal from the flow meter is input to a prover counter. The prover counter then calculates the correction factor for the meter.
In a separate activity, the prover is itself calibrated by a volumetric water measurement called water draw. During water draw the temperature and pressure of the water are noted. The volumetric water draw thus measured is corrected to a standard reference temperature and pressure both for the water and for the material of construction of the prover to obtain the water draw volume.
Prior art provers have design features which may contribute to inaccuracies in proving, may develop undetected leaks, and may cause seal leaks and other maintenance problems.
When provers are used to calibrate meters, the fluid temperature and pressure are noted and used to calculate volumetric corrections. These corrections are necessary because the actual prover volume will be different from the water draw volume if the actual temperature and pressure are different from the standard reference water draw temperature and pressure. However, these corrections are inaccurate to the extent that the assumptions of the calculations are inaccurate. For example, it is assumed that the measuring chamber has a uniform wall thickness and that it is unrestrained from expanding due to temperature or pressure increases. It is assumed that the walls of the measuring chamber are uniformly at the fluid temperature. For most of the provers in use, these assumptions are inaccurate to a greater or lesser degree. So, in practice the volumetric corrections are inaccurate.
An obvious solution to this problem is to have flow fluid inside and outside the measuring chamber and to keep the measuring chamber unrestrained from thermal expansion. This is known as double wall construction. Double wall construction has traditionally been used to maintain internal temperatures and to balance pressures. Numerous examples can be found in nature and in the continuum of human design from the earliest to the present. It is the application of this principle to the solution of specific problems that requires new art.
2. Prior Art
Howe U.S. Pat. No. 3,273,375, issued Sept. 20, 1966 discloses calibrating apparatus utilizing a double wall construction in which the measuring conduit is enclosed in an outer housing. An object of this type of construction is to substantially reduce stress and strain on the walls of the measuring chamber. However, Howe shows the measuring conduit supported by ribs along its length. These ribs transfer strain from the outer housing to the measuring conduit, distorting the measuring chamber and substantially reducing the benefit of the double wall construction. Further, the ribs prevent fluid contact at the outer surface of the measuring chamber and also transfer thermal energy from outside the apparatus to the measuring chamber. These conditions cause thermal distortions.
Francisco U.S. Pat. No. 3,492,856, issued Feb. 3, 1970 discloses a prover apparatus with an envelope forming an annular space, communicating with the flow fluid, around the measuring chamber. But, the cylinder is attached at the opposite end of the fluid conduit shell from the rod extension bearing and seal creating great difficulties in obtaining and maintaining alignment.
Waugh and Wehrli U.S. Pat. No. 4,372,147, issued Feb. 8, 1983 discloses a prover apparatus with a double wall construction similar to the Francisco apparatus except the measuring chamber is rigidly mounted to a blind flange at the rod extension bearing and seal or downstream end. In the Waugh design, it is necessary to provide a fluid barrier in the annular space between the cylinder and the outer housing. Waugh teaches that this can be done in a manner permitting free expansion of the measuring chamber, but, in practice, this has not been done because of conflicting needs to provide a double block and bleed seal and also to provide additional support at the outer end of the cylinder. This introduces all of the defects of the Howe and Francisco designs. There is thermal conduction and distortion. There is restraint against free radial expansion. There are serious alignment problems. There is insulation of the measuring chamber, from the fluid temperature, introducing strain and distortion.
Since the piston reciprocates in the cylinder and the flow through the meter cannot be stopped or reversed, there must be a provision for returning the piston after the prove stroke. This introduces many difficult problems. An alternate flow path must be established before the piston comes to rest at the end of the prove stroke, while the piston remains at rest, and while it is being returned upstream against the flow.
There is another class of compact prover, the bidirectional of which the Howe design, cited above, is a member which attempt to solve the problem by using a four-way by-pass valve, but this introduces more problems than it solves. Provers of this class simply reverse the flow through the fluid conduit so as a result the piston never has to be returned against the flow. Reversing the flow is done with a four-way valve. Besides being bulky, costly, and slow, these valves have a multitude of other problems such as short seal life and seals whose integrity is not easily verified. These provers tend to be large, expensive, and unreliable.
The Francisco design cited above and his U.S. Pat. No. 4,152,922, issued May 8, 1979, both disclose a design in which the piston has a poppet valve which opens at the end of the stroke and remains open while the piston is being pulled back upstream. There are several problems with using a poppet valve in the piston. To get a reasonable size flow path through the poppet valve it tends to be large in diameter making the piston large in diameter, and, thus making the prover large in diameter. Because it takes an interval of time for the poppet to close and to open, and because it must be closed when the detectors indicating each end of the measuring chamber are crossed, there must be an additional length on each end of the cylinder for the valve to open and close, thus making the prover longer. The poppet valve can be closed either with a spring or with differential pressure. Differential pressure is very undesirable when the poppet is closed during the prove stroke so, in practice, the valve is closed with a spring. But then it must be opened with differential pressure during the return stroke and this is undesirable also. Differential pressures across the prover cause flow disturbances which introduce inaccuracies into the meter proving.
The Waugh patent cited above discloses a design in which a by-pass valve is used to route the flow through a by-pass conduit when the cylinder is blocked by the piston. The by-pass valve is closed during the piston's downstream prove stroke and opened during the upstream return stroke. Provision must be made for flow continuation during the interval at each end of the stroke when the piston is stationary and the by-pass is closed. Waugh accomplishes this by providing a set of apertures beyond each end of the measuring section of the cylinder but inside the piston's limiting upstream and downstream positions. This not only adds considerable length to the cylinder, but also adds potential damage to the seals from riding over the apertures. Also it is costly to install apertures around the periphery of the cylinder.
In prover apparatus, it is highly desirable to approach zero differential pressure across the piston during the prove stroke to reduce flow perturbations at the very time flow is being measured. To try for zero differential piston pressure, there are three principle forces to balance. First is the force of friction on all sliding components. Second is the force of the flow fluid pressure acting on the differential area of the piston--effectively the pressure force on the rod end inside the prover. Third is the force of the control fluid pressure acting on the end of the rod outside the main conduit.
The Francisco design, cited above, discloses that during its prove stroke the rod is entering the prover. Thus the first and second forces act in concert to slow the piston down, thus again, the third force must balance both the first and the second force. This design is complicated by the fact that the rod end outside the prover has an auxiliary piston inside an auxiliary cylinder with one fluid resisting motion and the other aiding motion during the prove stroke. None of these forces are strictly constant and balancing them becomes a complicated act indeed. If the flow fluid pressure rises, the rod is in danger of failing as a column. Further, Francisco teaches that a differential pressure across the piston aiding in keeping the poppet valve closed is desirable.
The Francisco patent also disclosed a modification in which the piston rod extends out both ends of the prover. This effectively eliminates the second force but adds additional friction forces.
The Waugh design, cited above, discloses that during its prove stroke the rod is exiting the prover. Thus, the first and second forces act opposed. The third force, which in the Waugh apparatus is a singular pressure acting on the exterior end of the rod, therefore, must only balance the net of the first and second forces. The net of the first and second forces may resist or aid the motion of the piston during the prove stroke. If the net resists the motion of the piston, the third force, which, being pressure, can only act in compression, is unable to balance it. In that case an undesired differential pressure develops across the piston. If the net of the first and second forces aids the motion of the piston, then it-can be balanced by a control fluid, usually a hydraulic oil system.
Controlling the motion of a rod with controllable fluid pressure or flow is an old art. The means of that control may be new art. Waugh discloses two means of controlling the control fluid flow during the prove stroke. In one scheme Waugh regulates the flow of the control fluid in response to the flowing fluid pressure on the upstream side of the piston. In another scheme, Waugh regulates the flow of the control fluid in response to the differential pressure of the flowing fluid between the upstream and downstream sides of the piston. Both of these means require elaborate and expensive controllers which are subject to error and failure.
These methods for controlling piston motion are quite complicated. The first two forces acting on the piston are constant except during a brief moment at each end of the stroke when it is outside the measuring chamber anyway. During the prove stroke, in the measuring chamber, the friction is virtually constant and the fluid flow pressure is nearly constant. Therefore, the control pressure need only be constant if it is correct.
The systems in which provers are used run between two extremes in regard to their flow-pressure relationship: hard systems and soft systems. In hard systems, large differential pressures across the prover result in very small flow rate changes. In soft systems, a small differential pressure across the prover results in a large flow rate change. In both systems, the piston should move downstream in compliance with the flow during the prove stroke. Since achieving perfection is difficult, there will usually be a small differential pressure across the piston. In a hard system, if the controlling fluid is not compliant, that is, pressure controlled, the tendency of the piston to match its speed to that of the flow fluid may be resisted, the differential pressure may rise, and a flow disturbance may result. In a soft system, if the controlling fluid is not resistant, that is, flow controlled, the tendency of the piston to accelerate because of the differential pressure will be accommodated and the piston will change speed, unrestrained by the fluid flow, and will cause a flow disturbance. None of the prior art accommodated this.
In addition, prior art provers of the type disclosed by Waugh, followed the natural tendency of making the piston rod as small as possible, consistent with not failing as a column. But, this meant that in low pressure systems, the second force could never be large enough to balance the first force and so the controlling fluid, able to only act in compression, could not control.
Means must also be included in compact provers for stopping the piston at the end of the prove stroke. At high flows the speed is considerable and the piston, particularly with the rod attached, as a significant amount of kinetic energy. If the piston is stopped at the end without consideration for an interval of deceleration it will be subject to damage and will emit objectional noise. The Howe apparatus employs rods projecting from the center of both ends of the outer housing to stop the piston motion. The Francisco apparatus discloses a portion of the poppet valve associated with the piston assembly which is designed to directly contact the downstream end of the cylinder to operate the poppet valve. These prior art apparatus rely on unyielding metal to metal contact to stop the piston. The Waugh apparatus discloses a third set of apertures provided in the conduit downstream of the second set of apertures, whereby, when the piston covers the third set of apertures, the fluid within the conduit smoothly decelerates the piston motion. These holes are fixed and cannot be adjusted to meet changing conditions. Again, holes of this type are costly, can cause seal breakdown both from a grating action and from hydraulic action when the seals are forced outward against the apertures from pressures and flow at high rates of energy absorbtion.
During the prove stroke, if there is any fluid leakage between the upstream and downstream sides of the prover, the calibration will be inaccurate to the extent of the leakage. The Francisco apparatus has no method of testing for leakage during operation, but, rather requires that the flow through the apparatus be stopped and a lengthy testing procedure be followed. It is hoary art in this field to employ two seals with a space between that can be drained to demonstrate no leakage conditions. This is called a double block and bleed system. There are many valves in the trade that employ this principle. Most cylinders with pistons employ the double block principle. Francisco's U.S. Pat. No. 3,492,856 does so. Shepherd et.al U.K. Patent Application G.B. No. 2,088,566A filed Nov. 28, 1980, shows double block piston seals with an annular space between, he also teaches, in one embodiment, the rod is hollow to provide communication from the space between the seals to a pressure sensor carried on the end of the rod so as to provide a means of monitoring the efficacy of the seals while the piston is in the cylinder. Pfrehm GB Patent Specification No. 1,275,639 published May 24, 1982, also shows double block piston seals, with an annular space between. The Waugh patent discloses the double block piston seals, an annular cavity formed between the seals, a passage connecting the cavity to one end of a flexible tube, and means for connecting the other end of the flexible tube to the exterior of the apparatus, whereby the integrity of the piston seals may be continuously monitored from the output of the flexible tube while the piston is stationary or in motion. Waugh also teaches that the integrity of the seals on the by-pass valve may be similarly monitored. In practice these seals are monitored by opening the passage from the space between the seals so that the space between the seals freely communicates with space outside the prover. Flow fluid from between the seals is drained or blown off thus reducing the pressure between the seals. The passage is then closed and the pressure is monitored to indicate any leakage past the seal. This procedure can be manual or automatic, both are used. This procedure creates several problems. Some flow fluid is wasted or must be recycled and recovered. Unless precautions are taken, some flow fluid escapes to atmosphere which practice is often undesirable. In high pressure fluid flow systems, a large differential pressure can be created between the flow conduit and annular space between the seals which can either damage the seals or, for seals which tighten with increased differential pressure, may lock the seals against the inside of the cylinder impeding piston motion. Separate tests are always performed on the piston seals and the by-pass valve seals which require additional time. The seal between the rod and the piston, which is hidden from testing, does not get an integrity check although it is desirable to do so.
In proving, the volume displaced by the piston stroke must be measured. The Francisco U.S. Pat. No. 3,492,856 Feb. 3, 1970 discloses means for indicating the angular displacement of the drum which is related back to the displacement of the piston through a cable. There are many accuracy and seal problems with an angular detector and cable.
The Pfrehm et al prover, the Francisco U.S. Pat. No. 4,152,922 prover, and the Waugh apparatus all disclose two detectors in a spaced apart relationship where the distance between the first and second detection means corresponds to the length of the fluid measuring portion of the conduit, and with only one characteristic on the piston or rod being detectable. If either of the detectors should fail and be replaced the distance between the detectors can change significantly, requiring a new water draw. It is also known in the art to use a photoelectric cell to detect indicia.
The detectors used are various. Magnetic limit switches, photocells, induction devices are some examples. They each have either an accuracy or reliability problem or require a complex support system.
The Shepherd et al patent discloses a method of proving in which the meter signals define the start and stop of the prove stroke, the object being to prove the meter over integral numbers of meter revolutions. It has long been known that this was desirable since the meter flow-to-signal relationship is not constant over a revolution of the meter. Shepherd also discloses a meter prover in which a part attached to the rod cooperates with an encoder to provide signals denoting increments of movement of the piston. This prover is very difficult to carry out in practice because some awkward means, of the type Shephard shows in his drawings, must be found to control the piston. Thus, such a prover seems impractical.
All prior art provers are constructed with one or more pipe flanges used to connect the fluid flow conduit. The prover must be disassembled to gain access for maintenance, and for other reasons. There are numerous problems associated with these flanges including that they are assembled with numerous fasteners requiring prolonged labor to assemble and disassemble.
All unidirectional compact provers have a rod or cable extending from the end of the outer housing or flow conduit. Because of the means of construction there is excessive bearing wear and seal failures. Typical means of construction is to mount the control fluid pressure chamber rigidly to the outer housing of the flow conduit. This then requires the accurate alignment of three widely spaced points on a welded structure which is very difficult at best.
For all compact unidirectional provers that have a rod or cable extending from one end, the quantity of fluid taken in differs from the quantity of fluid put out during the prove stroke because of the displacement of the rod or cable. Most flow systems in which a prover is used, have a flow restricting means as well as the meter and a pump or other flow inducing means. The flow restricting means, for example, might be a flow control, a partially closed valve, an orifice plate, or just a long run of pipe.
It is arbitrary practice to install the meter on one side of the prover and the flow restricting means on the other side of the prover. During the prove stroke, since the pressure at the flow restricting means will change very little, the flow rate through the flow restricting means will change very little. The meter meanwhile will simply follow whatever flow there is. Since the volume of flow that enters the prover is not the same as the volume of flow that exits the prover, the meter will speed up or slow down during the prove stroke; there will be a flow disturbance. Since disturbances can cause measuring error, this method of arranging the piping system is very undesirable.
In prior art provers the prover is always mounted with the prover axis horizontal. Some provers have been developed that can be tipped up and down between a vertical and a horizontal orientation, but this requires a very large support. Sometimes horizontal orientation is desirable to keep a low center of gravity. Keeping the downward projected area small is always desirable but the present methods of mounting present a large downward projected area.